Secondary recovery of oil



SECONDARY RECOVERY OF OIL Jan Offeringa, Amsterdam, Netherlands, assignor to Shell 5 Development Company, New York, N. Y., a corporation of Delaware No Drawing. Application September 15, 1955 Serial No. 534,616

Claims priority, application Netherlands September 17, 1954 1 Claim. (Cl. 166-2) The invention relates to a process for producing petroleurn from oil-bearing formations, according to which the oil is displaced by another medium, preferably a liquid such as water or an aqueous liquid.

Processes are known for producing petroleum by the use of natural or artificial water drive. In these processes Water or an aqueous liquid is forced under pressure into the formation as a result of which oil is displaced and afterwards produced via one or more production wells. It is also known to use a washing liquid as the displacing medium which has the property of loosening the oil from the rock. However, only a part of the oil present in the formation can be produced in this way.

It is an object of the present invention to provide a process for enabling a greater part of the oil to be displaced from a formation and afterwards produced.

According to the invention external forces opposite or approximately opposite to each other are alternately exerted during the displacing process on the oil to be displaced, so that the pressure gradient in the oil is thereby 3 reversed each time and the oil or part thereof is enabled to move to and fro, in such a way that (by a suitable choice of the intensity and duration of the forces) on an average calculated over the time, a net transportation of oil in one direction is obtained.

Model experiments showed that it is possible to obtain a considerably higher oil production under more favorable conditions with the process according to the invention than was the case in the same period with known displacing methods.

The favorable results of the present process and the mechanism on which this process is based may be explained as follows.

The porous formation containing the oil may be considered as a system of branched capillaries of different diameter. In displacing the oil, for example by means of an oil-immiscible liquid such as water, it will penetrate more readily and rapidly the capillaries with a larger diameter than those with a smaller diameter. In general there is even a minimum value for the external pressure which must be exerted on the displacing liquid if the latter is to penetrate a capillary of a certain diameter. This pressure is higher according as the capillary is narrower. In the known displacing methods, the oil is practically only displaced by the continued exertion of pressure from the widest capillaries. In the narrower capillaries the water is entirely unable to penetrate, or unable to penetrate so rapidly, and when the interface of water and oil in a wide capillary passes the connecting place with a narrow capillary (at the end of this capillary, measured in the direction in which the displacement oc curs) the oil is enclosed in the narrow capillary so that it remains behind in the formation.

According to the present invention, however, it is nevertheless possible to produce the oil enclosed in the narrow capillaries. If the direction of the external forces exerted on the capillary system is temporarily reversed, so that the pressure gradient is reversed, the interface of water 2,823,751 Patented Feb. 18, 1958 and oil is forced back again. The flow thus caused is,

however, not at all identical with the reverse of the forward movement, since capillary counter-pressures completely different from those in the first movement play a art.

The interface of oil and water moves in the narrow capillaries (as compared with the wider capillaries) with far more difiiculty than was already the case in the forward movement, and there is often no movement at all in the reverse direction. Thus, it is possible that after some time the interface in the wider capillaries is forced back to beyond the place of the interface in the narrow capillaries, even before the interface in the Wide capillaries is forced back to the initial position (the beginning of the forward movement). Afterwards the external forces may again be applied in their original direction. Thus the forerunning of the interface into the wider capillaries can be equalized in the manner indicated and a uniform progress of the interface promoted in all capillaries.

If necessary, the above procedure can be repeated one or more times, as a result of which the different interfaces reach the connecting place of the capillaries more or less simultaneously so that in principle the oil can be displaced from all capillaries.

The intensity and duration of the external forces to be used should, of course, be chosen in such a way that on an average, calculated over a fairly long period, a net transportation of the oil is obtained. For example, a duration of the reverse movement may be chosen which is half that of the forward movement, assuming equal (but opposite) external forces in both cases. However, different intensities of the forces in both cases may, of course, be chosen, or a suitable combination of intensity and duration of the external forces may be used for the forward and reverse movement.

In the present process liquids are preferably used in the displacing medium; usually water or aqueous liquids are used for the purpose. Certain substances, such as surface-active substances, salts, etc. may be added with the water. Generally, any liquids known as displacing media may be used.

Occasionally water is not injected directly, but first a more or less oil-miscible liquid (e. g. a solvent for oil) followed by the liquid which finally has. to take the place of the oil in the formation. In this case also the process according to the invention may be used. The present invention may even be used when the solvent is being injected. Sometimes gases may also be used as displac ing media. If necessary, the displacing medium can be injected at a high temperature.

The process is generally carried out with the use of one or more wells for producing the oil and one or more wells for injecting the medium. The desired movement of the oil to be displaced may then be obtained by choosing a pressure in the production well(s) which is alternately higher and lower than the pressure in the injection well(s); both superand sub-atmospheric pressure may be used. The mean pressure of the injection and production well may be higher or lower than or equal to the atmospheric pressure.

Super-atmospheric pressure in the injection well may, for example, be obtained by injecting displacing liquid or gas; the super-atmospheric pressure in the production well may, for example, be obtained by injecting gas or oil (e. g. produced liquid).

If two (or more) production wells are available the process may be carried out in such a way that one well is reserved for the actual production, the other being used for exerting the reverse external pressure on the oil (the first well is then closed). In this case this external force will often have a direction which is not exactly opposite to the direction of displacement, but varies somewhat therefrom.

It is possible to carry out the displacement process in a more or less vertical direction instead of in the usual horizontal direction. In this case the same well may be used as an injection and production well at the same time, the injection of the medium and the production of the oil displaced occurring at different heights in the oil-bearing formation (usually at the top and at the bottom, or vice versa).

The process according to the invention will be further illustrated by means of description of a model experiment carried out in a horizontal tube 160 cm. long. The tube is filled with sand with a permeability of 200 Darcys. This sand was saturated with oil having a viscosity of 300 cp.

Each end of the tube was in communication with a vessel. One vessel (A) was filled with water injected into the tube by means of air under 2 atm. gauge. As a result the oil was displaced from the sand; this oil was collected in the other vessel B) which was in communication with the atmosphere during the injection of water.

After water had been injected and oil produced in this way for some time, vessel B was brought under pressure by air under 2 atm. gauge and vessel A opened to the atmosphere. In this way a quantity of the oil already collected in vessel B was re-injected into the tube, while a quantity of water was again displaced from the tube into vessel A. This procedure (according to the invention) was repeated a number of times. The super-atmospheric pressure in vessel A was maintained on each occasion for one minute, and the super-atmospheric pressure in B /2 minute.

By choosing a smaller period in which oil (afterwards oil and Water) was displaced than the period in which oil (afterwards oil and water) was produced, a net yield of oil, and afterwards of oil and water, could be obtained.

At the moment of the water break-through into vessel B, 19% of the oil originally present in the tube was produced, while with a corresponding test without an intermittent reversal of the direction of flow only 10% of the oil could be displaced.

With a cumulative production (oil and water) of 65% of the pore volume of the sand (total duration 90 minutes), a yield of 25% of oil was obtained when using the above process according to the invention. With the known, direct displacement only 15% of oil was produced.

In order to give an idea of a suitable duration in practice of the external forces, it can be further stated that a period of a minute in the model experiment approximately corresponds to a period of from some weeks to some months in the practical operation of the process.

In practice the process may often be successfully employed in lenticular oil fields; these fields usually have a finer structure than the surrounding layers. An increased oil production can be obtained when according to the invention the encroaching flank water is allowed to rise and is forced back again at intervals. Thus, the movement of the oil in the direction of displacement is in this case wholly or partly caused by a natural water drive.

I claim as my invention:

A method of recovering hydrocarbons from a hydrocarbon-bearing formation that is substantially devoid of natural fluid energy and that is penetrated by at least one input well and one output well, .said method comprising injecting an aqueous flooding fluid under pressure into the formation through the input well so as to drive hydrocarbons in said formation toward said output well at a given pressure gradient and produce them at the output well, reversing the pressure gradient in said hydrocarbon-bearing formation by stopping the injection of aqueous fluid into said input well and injecting the produced fluid under pressure into said output well so that at least a portion of the hydrocarbons in said formation moves temporarily in a direction substantially opposite to that prevailing during normal injection, and repeatedly reversing the pressure gradient in said formation so that the hydrocarbons therein are caused to move back and forth in the formation, the intensity and duration of the movement of hydrocarbons in each direction being such that there is a net movement of hydrocarbons toward said output well from which all of said hydrocarbons are produced.

References Cited in the file of this patent UNITED STATES PATENTS 1,295,243 Waitz Feb. 25, 1919 2,115,378 Wolf Apr. 26, 1938 2,115,379 Wolf Apr. 26, 1938 2,347,778 Heath May 2, 1944 

